Lactic acid is broadly used for application in food, and for application in chemical raw materials such as of medical treatments and cosmetics. Also, polylactic acid which is obtained by using lactic acid is drawing attention as a biodegradable plastic which is degraded finally into carbon dioxide and water by a microorganism and the like. Because of this, it is necessary to produce lactic acid with a low cost and a high productivity.
As the method for producing lactic acid, there is known a biological method in which it is produced by fermenting a sugar by a lactic acid bacterium. However, since lactic acid bacteria have low acid resistance, in order to obtain a high productivity by this method, it is necessary to convert the lactic acid produced by the fermentation into a lactic acid salt by neutralizing it with an alkali. Since such the neutralization with an alkali requires a step for restoring lactic acid from the lactic acid salt, the production process becomes complex and the production cost also becomes high.
As a method for obtaining lactic acid without carrying out neutralization with an alkali, there is a method using a transformant in which a gene encoding a lactate dehydrogenase is introduced into a yeast. For example, PTL 1 discloses that lactic acid can be produced with high productivity without carrying out a neutralization step with an alkali by culturing a transformant in which a lactate dehydrogenase gene derived from a mammal such as a human is introduced into S. pombe, and a part of a gene cluster encoding a pyruvate decarboxylase in the S. pombe host is deleted or inactivated. Further, PTL 2 discloses that lactic acid can be obtained by culturing a transformant in which an L-lactate dehydrogenase gene of Lactobacillus plantarum is introduced into Saccharomyces cerevisiae which essentially does not produce ethanol when it is cultured in a culture medium.